Unmanned aerial vehicles (UAVs) require range and durability in the air. To help achieve this, engines and their control systems for such UAVs need to be relatively light in weight and yet provide high performance.
One way to achieve lightness in weight is to omit as many non-essential components and control system parts as possible and/or to make such components lightweight. However, to survive in the extreme ambient conditions that UAVs are exposed to, the engine and its control systems also need to be robust and reliable.
Engine timing position sensing is an essential part of engine control. Without sensing of the rotational position of the crankshaft, camshaft or related rotational components, the UAV engine will either not start or will not continue to run.
In modern engines, rotational position of the crankshaft or camshaft is typically sensed by a fixed position Hall Effect, inductive or optical sensor detecting a timing indicator on a rotary member attached to the crankshaft or camshaft.
The crankshaft and/or camshaft position sensor arrangement is used to monitor the relationship between the pistons and various cylinder ports of the engine (i.e. for a two stroke engine configuration) or the relationship between the pistons and valves in the engine (i.e. for a four stroke engine configuration). The signals from the sensor are used to synchronise the timing of such pistons, valves and injection of fuel into the combustion chamber(s) and the ignition thereof. Crankshaft/camshaft rotational position sensing is also commonly used as the primary source for the measurement of engine speed in revolutions per minute.
Common mounting locations for the position indicator include the main crank pulley, the flywheel, the camshaft or on the crankshaft itself.
The crankshaft or camshaft timing position sensor is a vitally important sensor in modern day engines. When it fails, the engine will not start or will cut out while running.
Typically the position sensor arrangement includes a rotary encoder, having a disc or a toothed wheel or other notched indicator rotating in fixed relationship with the crankshaft or camshaft. A sensor detects one or more marks/teeth/notches and the signal generated is used to provide an accurate indication of the rotational position of the crankshaft/camshaft (and therefore of the connected pistons) for engine injection, ignition and combustion timing purposes.
Whilst it may be desirable to omit components and control system parts to provide weight benefits in certain engines, minimising components in engine systems, particularly where this may impact on any redundancy capability, can however lead to an increased risk of engine failure, and ultimately potentially complete loss of the UAV.
UAVs need to be reliable because the consequences of mechanical or electrical failure (such as engine mechanical failure or engine sub-system failure e.g. the engine timing system) can be very severe. If a UAV in some way fails whilst flying, this can result in the ultimate demise and destruction of the UAV when it falls back down to the ground.
Also, such failure can happen when the UAV is a long way from its base and potentially over dangerous or difficult terrain, making safe recovery risky or impossible. There is also the associated risk of losing very expensive surveillance or other equipment carried by the UAV.
The need for reliability becomes particularly crucial if UAVs are used in civilian areas where a failure could endanger human lives and personal property.
To maintain reliability, additional and/or more robust components can be used. However, this tends to lead directly to increased UAV weight and also additional unit and replacement costs.
Robustness of components tends to require more material or features in order for the components to better withstand shock, vibration, extremes of temperature and changes in temperature. This need for robustness of components is all the more relevant for UAVs which need to cope with harsh environmental conditions experienced at high altitude for long periods of time.
Furthermore in terms of reliability, having only one system on-board does not allow for any failure at all of the engine timing system. This may also not satisfy certain regulatory requirements for such aerial vehicles when used in specific applications.
It would hence be beneficial to have full redundancy in the engine timing system for UAV engines. This could obviously be achieved by exactly duplicating primary engine timing components, such as providing a second rotary encoder and sensor arrangement.
A benefit of such duplication of the primary engine timing system is that components are exactly the same between the two systems, making spare parts and maintenance easier to manage.
Furthermore, replicating the primary engine timing system with an entire range of the same engine timing components will enable full performance of the UAV to be maintained in order to complete a flight without returning to point of origin.
However, merely duplicating the primary engine timing system can significantly impact desired weight and space constraints of the UAV which can in turn reduce performance and range of the UAV. Such duplication can also result in an increase of the overall cost of UAV.
For example, for engines utilising a rotary encoder having a toothed/notched timing indicator wheel on an end of the crankshaft and a fixed sensor (such as a magnetic or optical sensor) to detect when a timing indicator on the wheel passes the sensor, adding a second such timing indicator wheel and sensor arrangement (or at the very least a second sensor detecting the timing indicator on the first wheel and having electronics to compensate for the different rotational position of the timing indicator) would add significant complexity and weight to the UAV.
Also, duplicating the primary engine timing system components invariably adds to the cost of the UAV engine systems, not least because the primary engine timing system components needing to be robust are therefore relatively expensive.
Whilst it would be beneficial to have full engine timing system redundancy with two matching (duplicated) engine timing systems for UAV engines, the added weight and space of the complete second system would be too detrimental to performance and the effect on cost would also be an issue. That is, an engine timing system along these lines which includes full duplicated standard engine timing systems, one serving as a primary system, and one for backup/redundancy purposes, would make for an uncompetitive overall engine package.
The need to provide for redundancy of key engine components and control systems is however required by civil aviation authorities for many aircraft systems.
As discussed above, the mere duplication of parts to provide such redundancy capability in such UAV aircraft systems is in conflict with the requirement to keep weight to a minimum. Accordingly, an engine timing system which can provide partial redundancy operation is proposed, which fulfils the need for operational redundancy without incurring a significant weight (and cost) penalty incurred by such direct duplication.
Systems have previously been proposed that provide a backup for failure of timing position sensing or other absence of a timing position signal for an engine. However, these typically require either duplication of components to provide redundancy, or have additional components that are used to emulate a timing signal or help derive a timing signal. Such systems do not provide for partial redundancy of an engine timing system which fulfils the need for operational redundancy without incurring a significant weight (and cost) penalty incurred by direct duplication of timing system components or adding other components that are used to emulate or replace components in order to generate a replacement timing signal, and thus add significantly to weight and cost.
US 2007/0256482 discloses a system which alleviates electromagnetic disturbances when a ‘bumpless’ crankshaft positioning system of a railway locomotive engine switches over from a failed crankshaft position sensor to a backup crankshaft position sensor. A second crankshaft position signal stream is generated to compensate for any missing or false crankshaft position signals (e.g. when a toothed crankshaft timing wheel has a missing tooth). That is, the second crankshaft position signal stream emulates the first crankshaft position signal stream. To achieve this, the system of US 2007/0256482 either generates a virtual second crankshaft position signal stream from the first crankshaft position signal stream during a window of operation, or uses a second crankshaft position sensor to generate the second crankshaft position signal stream. Thus, US 2007/0256482 adds a second crankshaft position sensor and associated connections and processing to generate the emulated second crankshaft position signal stream, which adds weight and cost to the engine/vehicle.
U.S. Pat. No. 4,782,692 discloses determining simulated crankshaft angle signals by calculating the engine speed based on the elapsed time between two consecutive piston top dead centre positions of the crankshaft as sensed by a ‘top dead centre’ (TDC) crankshaft sensor mounted in an engine driven solenoid fuel pump. Based on the calculated engine speed, a program uses a lookup table or an algorithm to obtain a value representing the increment of time required for the crankshaft to rotate through a predetermined crankshaft angle. This value replaces a failed or missing crankshaft position sensor value. Accordingly, U.S. Pat. No. 4,782,692 requires not only the crankshaft position sensor but also the TDC sensor. As described hereinafter, the present invention proposes at least one entirely different methodology, system and solution to the problem of generating a backup timing signal,
U.S. Pat. No. 5,060,614 discloses a reference position sensor and an angle position sensor on the crankshaft of an engine. The reference position sensor provides an indication of cylinder position each revolution. The angle position sensor provides an indication of crankshaft angle. A cylinder discriminating sensor is provided on a camshaft. When each cylinder reaches a particular position, and the crankshaft is at a particular angle, a narrow pulse is produced, and when the reference cylinder reaches a particular piston position, a wide pulse is produced. A CPU uses all three sensor values to determine ignition timing. As a backup, when the cylinder discriminating sensor is abnormal, the reference cylinder signal can be produced based on the output of the reference position sensor. Therefore, U.S. Pat. No. 5,060,614 requires multiple position sensors to cater for redundancy in any one of them. This clearly adds cost and weight to the engine. The present invention avoids the significant weight (and cost) penalty incurred by duplicating sensors or having multiple sensors providing backup for each other.
JP 2009-250181 discloses a backup arrangement to deal with failure of a rotary encoder. The rotary encoder is arranged on a crankshaft. An auxiliary timing signal generator is provided to generate an auxiliary timing signal of a fixed frequency. If the rotary encoder fails, rotation angle of the crankshaft is determined as one pulse of the auxiliary timing signal based on a reference signal. JP 2009250181 requires the auxiliary timing signal generator to provide a backup timing signal as a redundancy measure. The present invention avoids such additional weight and cost whilst providing partial redundancy operation to fulfil the need for operational redundancy.
U.S. Pat. No. 7,621,176 discloses determining an absolute crankshaft angle of a power tool. A pressure sensor detects an operating pressure in the engine. An output of the pressure sensor and of a signal generator are linked together to determine the absolute crankshaft angle.
U.S. Pat. No. 7,974,767 discloses selecting a period duration of a voltage signal to correspond with the n-th portion of a crankshaft revolution. The n-th portion provides a crankshaft angle interval. For each crankshaft angle interval at least one detail is detected which represents a course plotted against the crankshaft angle. The course is scanned with regard to characteristic features that are correlated with an operating parameter of the power tool.
U.S. Pat. No. 7,974,767 seeks to determine operating parameters of an engine of a power tool. For example, the load on the tool, the operating state of a heater, the operating state of a valve, the wear state of the tool, crankshaft position, throttle position and fuel mixture, can be determined without sensors. In essence, U.S. Pat. No. 7,974,767 identifies the signal of an alternating voltage in order to derive therefrom an operating characteristic of the power tool, such as identifying a load signature or faulty component leading to an irregular or unexpected signal.
With the aforementioned in mind, it is desirable for the present, invention to provide engine timing system redundancy for a UAV that provides such redundancy capability but alleviates the problem of adding significantly to the weight and cost of the UAV.